(a) Technical Field
The present invention relates to an idle stop-start control method of a fuel cell hybrid vehicle. More particularly, the present invention relates to a fuel cell stop-restart control method for improving fuel efficiency and increasing the amount of regenerative braking in a fuel cell-storage means hybrid vehicle including a fuel cell as a main power source and a storage means (e.g. supercapacitor or battery) as an auxiliary power source.
(b) Background Art
A fuel cell is an electricity generation system that does not convert the chemical energy of fuel into heat by combustion, but electrochemically converts the chemical energy directly into electrical energy in a fuel cell stack. Such a fuel cell can be applied to the supply of electric power for small-sized electrical/electronic devices such as portable devices, as well as to the supply of electric power for industry, homes, and vehicles.
At present, the most preferred fuel cell for a vehicle is a polymer electrolyte membrane fuel cell (PEMFC), also called a proton exchange membrane fuel cell, that preferably comprises: a membrane electrode assembly (MEA) including a polymer electrolyte membrane (PEM) for transporting hydrogen ions and an electrode catalyst layer, in which an electrochemical reaction takes place, disposed on both sides of the PEM; a gas diffusion layer (GDL) for uniformly diffusing reactant gases and transmitting generated electricity; a gasket and a sealing member for maintaining airtightness of the reactant gases and coolant and providing an appropriate bonding pressure; and a bipolar plate for transferring the reactant gases and coolant.
In the fuel cell having the above-described configuration, hydrogen as a preferred fuel and oxygen (air) as a preferred oxidizing agent are supplied to an anode and a cathode through flow fields of the bipolar plate, respectively. The hydrogen is suitably supplied to the anode (also called a “fuel electrode”, “hydrogen electrode”, and “oxidation electrode”) and the oxygen (air) is suitably supplied to the cathode (also called an “air electrode”, “oxygen electrode”, and “reduction electrode”). The hydrogen supplied to the anode is dissociated into hydrogen ions (protons, H+) and electrons (e−) by catalyst of the electrode catalyst layer preferably provided on both sides of the electrolyte membrane. At this time, only the hydrogen ions are selectively transmitted to the cathode through the electrolyte membrane, which is preferably a cation exchange membrane and, at the same time, the electrons are transmitted to the anode through the GDL and the bipolar plate, which are conductors. At the cathode, the hydrogen ions supplied through the electrolyte membrane and the electrons transmitted through the bipolar plate meet the oxygen in the air supplied to the cathode by an air supplier and cause a reaction that produces water. Due to the movement of hydrogen ions occurring at this time, the flow of electrons through an external conducting wire occurs, and thus a current is suitably generated.
If the fuel cell is used as the only power source of an electric vehicle, the fuel cell powers all loads of the vehicle, which results in performance deterioration during operation where the efficiency of the fuel cell is low. Moreover, during high speed operation where a high voltage is required, a sufficient voltage required by a drive motor is not supplied due to a rapid decrease in output voltage, thus decreasing acceleration performance. Furthermore, if a sudden load is applied to the vehicle, the output voltage of the fuel cell suddenly drops and sufficient power is not supplied to the drive motor, thus decreasing vehicle performance (accordingly, a sudden change in load imposes a heavy burden on the fuel cell since electricity is generated by an electrochemical reaction). In addition, since the fuel cell preferably has unidirectional output characteristics, it is impossible to recover energy from the drive motor during braking of the vehicle, thus decreasing the efficiency of the vehicle system.
Accordingly, a fuel cell hybrid vehicle has been developed. Exemplary fuel cell hybrid vehicles include large vehicles, such as a bus, as well as small vehicles that are preferably equipped with storage means such as a high voltage battery or a supercapacitor as an auxiliary power source for suitably providing the power required for driving the motor in addition to the fuel cell as a main power source. At present, a fuel cell-storage means hybrid vehicle that does not employ a power converter has been studied, and the fuel cell-storage means hybrid vehicle has high fuel efficiency (e.g. high regenerative braking, high efficiency of supercapacitor, and without the use of the power converter), an increase in durability of the fuel cell, suitably high reliability control, and the like.
In the hybrid vehicle in which the fuel cell and the storage means are preferably directly connected, the fuel cell continuously outputs power at a suitably constant level during driving. If electric power is suitably sufficient, the storage means is charged with surplus power, whereas, if the electric power is insufficient, the storage means supplies the insufficient power to drive the vehicle.
An exemplary configuration of a fuel cell-supercapacitor hybrid vehicle is described below
FIG. 1 is an exemplary configuration diagram showing a power system of a fuel cell-supercapacitor hybrid vehicle preferably including: a fuel cell 2 suitably used as a main power source, a supercapacitor 10 suitably used as an auxiliary power source, a supercapacitor precharge unit 9 preferably interposed between a main bus terminal 3, which is an output port of the fuel cell 2, and the supercapacitor 10, and a motor control unit (MCU) (with an inverter), which is a power module for rotating a drive motor 8, connected to output ports of the fuel cell 2 and the supercapacitor 10, producing a 3-phase pulse width modulation (PWM) by receiving direct current therefrom, and controlling the motor drive and the regenerative braking. In preferred embodiments, the supercapacitor precharge unit 9 is used only to charge the discharged supercapacitor during initial start-up.
The above-described fuel cell-supercapacitor hybrid vehicle uses the fuel cell 2 as the main power source, which preferably receives hydrogen from a hydrogen tank 1 and air from an air blower (not shown) to suitably generate electricity by an electrochemical reaction between hydrogen and oxygen in the air. The drive motor 8 and the MCU 7 are preferably connected to the fuel cell 2 through the main bus terminal 3, and the supercapacitor 10 is connected to the fuel cell 2 through the supercapacitor precharge unit 9 to provide power assist and regenerative braking. Moreover, a low voltage DC-DC converter (LV DCDC) 11 for power conversion between high voltage and low voltage and a low voltage battery (12V auxiliary battery) 12 for driving fuel cell balance-of-plant (BOP) components 16 are connected to the main bus terminal 3. Furthermore, an air conditioner 13 and a heater 14, which are operated by receiving high voltage power through the main bus terminal 3, are suitably connected to the main bus terminal 3.
The fuel cell BOP components 16 such as an air blower, a hydrogen recirculation blower, a water pump, etc. for driving the fuel cell 2 are connected to the main bus terminal 3 to facilitate the start-up of the fuel cell 2. Moreover, relays 4 and 5 for facilitating connection and disconnection of power and a blocking diode 6 for preventing a reverse current from flowing to the fuel cell 2 are provided in the main bus terminal 3.
Reference numeral 15 denotes a driver of the fuel cell BOP components 16, and 17 denotes a heater for supplying heat to facilitate cold start of the fuel cell 2.
In order to facilitate understanding of the present invention, the configuration of a fuel cell system will be briefly described. FIG. 2 shows an exemplary air supplier and an exemplary hydrogen supplier. As shown in the figure, dry air supplied through an air blower 28 is humidified by a humidifier 29 and supplied to a cathode of a fuel cell stack 2. Preferably, exhaust gas of the cathode, humidified with water generated from the cathode, is delivered to the humidifier 29 and used to humidify dry air to be supplied to the cathode by the air blower 28.
The hydrogen supplier preferably comprises two lines. The first line supplies hydrogen to an anode of the fuel cell stack 2 through a low pressure regulator (LPR) 23, and a portion of hydrogen at an outlet port of the anode is recirculated through a recirculation blower 24. The second line supplies hydrogen at high pressure to the anode through a valve 25 and an ejector 26, and a portion of hydrogen from the outlet port of the anode is recirculated and supplied through the ejector 26.
Moreover, hydrogen remaining in the anode directly passes through an electrolyte membrane without generation of electricity and reacts with oxygen in the cathode, which is called “crossover”. In order to reduce the amount of hydrogen crossover, it is necessary to suitably reduce the anode pressure during low power operation and increase the anode pressure during high power operation wherein the output of the fuel cell stack is suitably increased. To this end, the low pressure regulator 23 is used singly when low pressure is required, and hydrogen at high pressure is supplied by controlling the valve 25 when high power is required or during hydrogen purging. Accordingly, the higher the anode (hydrogen) pressure is, the more the amount of hydrogen crossover is suitably increased. Since the hydrogen crossover effects the fuel efficiency and durability of the fuel cell, it is necessary to maintain a suitably appropriate anode pressure. A hydrogen purge valve 27 is used to discharge impurities and condensed water in the anode, thus ensuring the performance of the fuel cell stack. Preferably, the outlet port of the anode is connected to a water trap 31 such that the condensed water stored in the water trap 31 is discharged through a valve 32 if the amount of condensed water reaches a predetermined level.
The driving mode of the hybrid vehicle including the fuel cell as the main power source and the supercapacitor (or a high voltage battery which is a secondary battery) as the auxiliary power source preferably includes an electric vehicle (EV) mode in which the motor is driven only by the power of the fuel cell, a hybrid electric vehicle (HEV) mode in which the motor is driven by the fuel cell and the supercapacitor at the same time, and a regenerative braking (RB) mode in which the supercapacitor is charged.
However, in fuel cell-supercapacitor hybrid vehicle the supercapacitor is automatically charged by the fuel cell, which thus restricts the regenerative braking. Accordingly, stopping the operation of the fuel cell during low power operation and during regenerative braking will overcome this restriction. Moreover, it is possible to improve the fuel efficiency by restricting the use of the fuel cell during low power operation where the efficiency of the fuel cell is low.
To improve the fuel efficiency, the fuel cell stop/restart process is considered, i.e., an idle stop-start control process, in which the power generation of the fuel cell is stopped and restarted (the fuel cell is turned on and off), if necessary, during driving of the fuel cell-battery or fuel cell-supercapacitor hybrid vehicle. The idle stop of the fuel cell during driving of the vehicle is suitably distinguished from the shut-down of the fuel cell system after the vehicle operation is finished. Accordingly, it is necessary to distinguish a control process for the idle stop of the fuel cell from a control process for the shut-down of the fuel cell system.
U.S. Patent Publication No. 20030118876 discloses a technique in which a relay switch, connected between a fuel cell and a supercapacitor, is turned off to disconnect the output of the fuel cell during low power operation, or if the voltage of the supercapacitor is above a suitably predetermined level, and the relay switch is turned on to connect the output of the fuel cell if an output required by the vehicle is increased or if the voltage of the supercapacitor is below a suitably predetermined level. In this technique, the relay switch of a main bus terminal for disconnecting the output of the fuel cell is turned on and off to achieve the idle stop/start, and accordingly a separate relay on/off control is required.
U.S. Pat. No. 6,484,075 describes a technique in which the fuel cell power supply is cut off by determining an idle state that is based on a wheel rotational speed, whether or not a brake is operated, a state of charge (SOC), an electrical load, and the like, and wherein the fuel cell power supply is restarted if a power storage unit is below a predetermined SOC. Here, the conditions for entering the idle stop are considerably restrictive (e.g. the idle stop is performed if the vehicle stopped, if the load is below a predetermined value, if the brake is in an operation state, and if the SOC is above a predetermined value). Moreover, a separate device such as a DC/DC chopper is required at the fuel cell for the idle stop, and the DC/DC chopper is directly connected to the supercapacitor during releasing the idle stop state after the DC/DC chopper is used to limit the current.
Accordingly, it is preferable to provide a method for stopping and restarting the power generation of the fuel cell by a suitably simplified control process, while improving the fuel efficiency of the fuel cell and increasing the amount of regenerative braking. It is preferable to provide a method for maintaining the fuel cell at an optimal state even in a non-power generation region since the durability of the fuel cell may be decreased if the fuel cell stop region is suitably increased.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.